Usually, for a high temperature heat resistant part of a boiler, a heat exchanger, or the like, an internally ribbed steel tube (rifled tube) with spiral ribs (protrusions) formed on the internal surface of the steel tube is used to improve a power generation efficiency. Since the internal surface of the internally ribbed steel tube has a larger surface area by the ribs formed on the internal surface, a contact area between water vapor passing through the inside of heated tube and the internal surface of the tube increases, while allowing turbulence to occur in a fluid containing water vapor, thereby enabling a heat exchange efficiency to be enhanced. With a recent tendency of increased capacity and higher temperature/higher pressure service for the boiler, the demand for the internally ribbed steel tube has increased rapidly.
To produce the internally ribbed steel tube, a seamless steel tube or an electric resistance welded steel tube is used as a blank tube, the blank tube is sufficiently softened as necessary, and then in a cold working process a drawing die and a plug, which has spiral grooves on its outer peripheral surface for forming ribs for the tube, are used to draw the tube.
FIG. 1 is an explanatory view for schematically illustrating a method for producing an internally ribbed steel tube by cold drawing. When a blank tube 3 is cold drawn, a plug 1 is inserted into the blank tube 3 in a concentric manner relative to a die 2 and the blank tube 3 while one end of the plug 1 is held by a mandrel 4, and the blank tube 3 is drawn in the direction indicated by a hollow arrow while allowing the plug 1 to be rotated.
At this time, the external surface of the blank tube 3 is reduced by the die 2. Meanwhile, the internal surface of the blank tube 3 is pressed and processed along spiral grooves 1a provided on the outer peripheral surface of the plug 1 so that spiral ribs 3a are formed on the inner peripheral surface of the drawn blank tube 3.
The plug 1 has a structure such that one end thereof is held by the mandrel 4, and the plug 1 can be rotated freely. The plug shape greatly affects the dimensional accuracy such as rib height and rib shape (especially, rib corner part and lead angle) of the internally ribbed steel tube, and the seizure occurs between the blank tube and the plug depending on the drawing conditions. Therefore, for the production of internally ribbed steel tube, a drawing plug which has spiral grooves of a predetermined shape on its outer surface has conventionally been used.
FIG. 2 is schematic views showing cross-sectional shapes of a spiral groove formed in the drawing plug to be used for the production of an internally ribbed steel tube. The schematic views show, in stages, cross-sectional shapes perpendicular to the plug axis line for a representative spiral groove among those shown in FIG. 1. FIG. 2(a) is a sectional view from A-A of FIG. 1, FIG. 2(b) is a sectional view from B-B of FIG. 1, and FIG. 2(c) is a sectional view from C-C of FIG. 1.
Usually, the drawing plug for the internally ribbed steel tube is configured so that as side walls 1aa as being opposed to each other and a bottom surface 1ab constitutes each spiral groove to provide a plurality of stripes of spiral grooves 1a in the outer peripheral surface of the drawing plug, the radius of curvature r of a corner portion, where each of the side walls 1aa meets a bottom surface 1ab, be sufficiently large on the plug front end part of plug shown in FIG. 2(c), and it become gradually smaller toward the plug rear end part as shown in FIGS. 2(c) and 2(b). With the drawing plug being configured in this manner, each rib is formed in a staged manner on the internal surface of the blank tube, so that the seizure is unlikely to occur.
FIG. 3 is another embodiment, schematic views showing the cross-sectional shapes of the spiral groove formed in the drawing plug used for the production of an internally ribbed steel tube. Similarly to FIG. 2, FIG. 3 shows, in stages, cross-sectional shapes perpendicular to the plug axis line for a spiral groove. FIG. 3(a) is a sectional view from A-A of FIG. 1, FIG. 3(b) is a sectional view from B-B of FIG. 1, and FIG. 3(c) is a sectional view from C-C of FIG. 1.
The drawing plug shown in FIG. 3 is configured so that as side walls 1aa as opposed to each other and a bottom surface 1ab constitutes each spiral groove to provide a plurality of stripes of spiral grooves 1a in the outer peripheral surface of the drawing plug, the radius of curvature r of a corner portion, where each of the side walls 1aa meets the bottom surface 1ab, is kept constant all the way from the front end part of plug to the rear end part thereof, while the plug decreases in diameter, for example, at a gradient of 3 degrees from the plug front end part toward the plug rear end part. This also allows ribs to be formed on the internal surface of blank tube in a staged manner at a constant deformation reduction rate, so that the seizure is less likely to occur (refer to Japanese Patent Application Publication No. 2001-179327).
Further, although the cross-sectional shape of drawing plug is not shown, it sometime happens to use a drawing plug in which, in addition to the gradual change in the cross-sectional shape of the spiral groove formed in the drawing plug as shown in FIGS. 2 and 3, both edges of a spiral groove ridge is rounded or chamfered linearly to reduce the area of contact between the top surface of groove ridge and the internal surface of blank tube at the time of cold drawing, thereby reducing frictional resistance occurring between the groove ridge top surface and the blank tube (refer to Japanese Patent Application Publication No. 2006-272392).